Give the JIT its own arena pool to avoid lock contentions.

Sharing it with the verifier and the class loader is not ideal,
especially at startup time.

bug:27398183
bug:23128949
Change-Id: I1b91663a13f6c5b33ad3b4be780d93eb7fe445b4
diff --git a/runtime/runtime.h b/runtime/runtime.h
index 8aac4ce..83e77d2 100644
--- a/runtime/runtime.h
+++ b/runtime/runtime.h
@@ -561,6 +561,9 @@
   ArenaPool* GetArenaPool() {
     return arena_pool_.get();
   }
+  ArenaPool* GetJitArenaPool() {
+    return jit_arena_pool_.get();
+  }
   const ArenaPool* GetArenaPool() const {
     return arena_pool_.get();
   }
@@ -669,6 +672,7 @@
 
   gc::Heap* heap_;
 
+  std::unique_ptr<ArenaPool> jit_arena_pool_;
   std::unique_ptr<ArenaPool> arena_pool_;
   // Special low 4gb pool for compiler linear alloc. We need ArtFields to be in low 4gb if we are
   // compiling using a 32 bit image on a 64 bit compiler in case we resolve things in the image